Additive layer manufacturing (ALM) is a process by which an article is produced layer by layer using 3D model data. The process may be used for producing prototypes, samples, models, tooling or full scale components.
A known ALM technique uses a power beam, such as a laser or an electron beam for example, to selectively sinter powdered material. A continuous bed of the powdered material is laid over a support member in a sintering chamber, and the power beam is used to selectively sinter the powder in a desired pattern by scanning across the surface of the powder bed. The power beam is controlled to produce the desired pattern according to cross-sections generated from 3D model data (e.g. a CAD file) of the article. The sintering chamber is typically heated so as to pre-heat the powder material in the powder bed to below the melting point of the powder material, thereby making it easier for the power beam to melt the powder in the selected regions.
After each cross-section is scanned and the melted powder has solidified, the substrate is lowered by one layer thickness (typically of the order of 0.1 mm) to prepare for growth of the next layer. Another layer of the powder is applied on top of the preceding layer in preparation for sintering. The process is repeated until the article is completed. Thus as the process proceeds, a sintered article is constructed, supported by unconsolidated powder. After the article has been completed it is removed from the substrate and the unconsolidated powder is typically recycled to produce another article by the same ALM process.
Selective sintering may be used to produce shaped articles from powders of some metals, ceramics, glasses and thermoplastic polymers.
Presently, the most commonly used thermoplastic polymer material for selective sintering ALM processes is Nylon because it has a re-solidification temperature significantly lower than its melting temperature. This means that the powder bed process can be conducted without requiring any support structures for the article being produced, making Nylon particularly suitable for rapid prototyping applications. However, for many engineering application Nylon is unsuitable. It has a relatively low strength, has a relatively low resistance to chemical attack, and is relatively susceptible to UV degradation compared with high-performance engineering thermoplastics. Furthermore, the relatively low glass temperature of Nylon means that it has a continuous service temperature (the highest temperature at which a material can function for an extended period of time without failing) of approximately 60-90 degrees Celsius. This makes Nylon unsuitable for many engineering applications.
Engineering plastics exhibit superior mechanical and thermal properties in a wide range of conditions. Some examples of high performance engineering thermoplastics currently used in the aerospace industry, which has particularly demanding material requirements, include polyetheretherketones (PEEK), polyetherketones (PEK), and polyetherimides (PEI). To a more limited extent acrylonitrile butadiene styrene (ABS) plastics are used. Whilst ABS plastics provide high strength, chemical resistance and good stiffness their use can be limited by relatively low operating temperatures compared with other high performance thermoplastics, such as those mentioned above.
There is a desire to produce high performance thermoplastic components using the selective sintering ALM process outlined above. However, there is currently a problem in that many high performance thermoplastics not only have a high melting temperature, and therefore require processing at higher temperatures, but they also generally have a re-solidification temperature only just below the melting temperature.
In the case of PEEK, a material which is particularly suitable for aerospace and medical device applications, the re-solidification temperature can be less than 1 degree Celsius below the melting temperature, which is approximately 375 degrees Celsius. It has recently been suggested to produce PEEK components by selective sintering ALM. However, the suggested approach is to maintain the powder bed in the sintering chamber at a temperature very close to the PEEK melt temperature, following process principles that apply for the much lower temperature Nylon material.
Producing an article by the ALM selective sintering process can take a long time. A typical build speed currently may be of the order of 0.1 kg/hr. Therefore, even relatively small articles can take several hours to produce. PEEK is susceptible to thermal aging due to shortening of the polymer chain lengths, and so maintaining PEEK material at high temperatures for extended periods of time has adverse consequences on its material properties. In particular, the fracture toughness of PEEK is significantly reduced by thermal aging.
To date it has not been possible to produce an article from PEEK by the ALM selective sintering process without significant degradation in the PEEK material properties. Moreover, the un-sintered powder material, which would normally be recycled for use in producing a subsequent article, will probably need to be discarded as the thermal aging effects of a further process would reduce the fracture toughness of the material yet further. The high material wastage adds significantly to the unit cost of each article produced.